Vitreous enamel composition



Un ted tar Pa n VITRE'OUS ENAMEL COMPOSITION Charles H. Commons, Jr.,Malvern, andWayne B. Shearer, Paoli, Pa., assignors to Foote MineralCompany, Phil adelphia, Pa, acorporation of Pennsylvania No Drawing.Application December 12, 1955 Serial No. 552,25 2 j 8Clairns. Cl. 106-48The present invention relates to a novel vitreous enamel composition;and, more particularly, the .invention re- Patented June 16, 1959 ice vwhich can be tired at moderate temperatures in the neighborhood ofl450-1500 F.

Still another object of the present invention is to provide a vitreousenamel composition possessing the characte'ristics discussed intheforegoing objects in the form ofa frit as well as in the form of thefinal enamel.

Other objects will become apparent from a consideration of the followingspecification and the claims.

"The vitreous enamel composition of the present invention is atitania-op'acified enamel frit consisting essentially of f certainspecific materials within certain well defined ranges "of proportions asfollows (all percentages by Weight): between about 11.2 and about 18.3%,of oxides of at least two of the alkali metals selected from the groupconsisting of sodium, potassium and lithium, from v 0 to about 7.8% ofat least one of the divalent oxides selected from the group consistingof CaO, BaO, MgO,

PbO, so, and ZnO, between about 3.9 and about 17% of H50 from 0 to about4.9% of P 0 between about 0.5 and about 10% of ZrO from 0 to about 4% ofA1 0 between about 37.5 and about 56.6% of SiO between about 1.8 andabout 6% of fluorine, and between about acid and alkali attack have beendeveloped. Such enam-' V els are dark in color and require relativelyhigh firing temperatures, in the neighborhood of 1600 1650 One suchacid-, and alkali-resistant enamel which has been suggested contains 3 258% SiO 616% ZrO,, 4-9% TiO 0-5% ZnO, 0'-3.5% 150 03.5% A1 0 644% B 013-20% Na O and 25-11% F27.

ever, the opacity of this enamel isfar below the present day standardswhich require a reflectanceof about 70% for a white enamel at about 25grams of dry enamel per square foot. This enamel just reaches thisfigure of reflectance at about 55 grams per square foot.

On the other hand, in the manufacture of enamels for ,fappliancesfl suchas refrigerators, stoves, washers, dryers, and the like, there arerequired, for esthetic appearance, enamels which are highly opaque,white and glossy, and which can be fired at moderatelylow temperatures,in the neighborhood of 1450-4500? F., to allow the use of lighter gaugemetals. In th'edeyelopment of enamels for the appliance field,acid-resistance aswell as high opacity have been considered of primeimportance.

More recently, however, with the use of stronger alkaline detergents,especially in. automatic dishwashers and clothes washers wherethefenamel. is subjected to hot water and alkali, alkali-resistancehasbecome a primary requisite. Unfortunately, the super-opaque andacidresistant appliance enamels have possessed poor alkali resistance.

It will be seen from thefforegoing that the presenflday enamels areseriously deficient in at least one of the three primary properties:super-opacity, acid-resistance and alkali-resistance. The industry hasbeen seekingan enamel possessing, in addition to these, the requiredwhite color, gloss and surface appearance and moderate firingcharacteristics.

It is the principal object of the present invention to provide avitreous enamel composition which, in the form of an enamel, isresistant to acid and to alkali and is highly opaque. r r r a 3 i It isanother object of the present inventiontoprovide a vitreous enamelcomposition which, in the'form of an enamel, is both alkaliandacid-resistant as well as highly: is

opaque and possesses the color, gloss and surface characteristicsrequiredfor. present dayuse on; appliances.

A further object of the present invention is to provide a. vitreousenamelcomposition not only possessing the ch c r c d u sed i t e for g gO jects, but also 16ar1d about 23% of TiO In the preferred compositionsthe oxide analysis will be as follows: combined alkali metal oxidesbetween about 13 and about 16%, divalent oxide between about 1.8 andabout 5.8%, B 0 between, about 7.9 and about 11%, P 0 between about 058and about 2.9%, Zr0 in an amount between about 2 and about 6%, A150 inan amount between about 0.5 andf about 2%, Si0 in an amount betweenabout 44 and 1abo1it 52%', fluorine in an amount between about 2.5 andabout 4.9% and 'liO in an amount between l8 and The vitreous, enamels ofthe present invention have been found to possess many outstandingcharacteristics. Principal of these are super-opacity, acid-resistanceand alkali-resistance. As mentioned above, present day requirements foropacity for white enamels dictate that the enamels possess a reflectanceof at least about 70% in applications of 25 grams of dry enamel persquare foot. The. enamel compositions of the present invention allpossess opacities higher than this, and the preferred compositionspossess substantially higher opacities, even as high as 85%, or more, atthe stated application rate. In many cases the present enamels possessopacities above "70% at application rates substantially lower than 25 4grams per square foot of surface.

" milligrams per square inch. 1

Referring to acid-resistance, an enamel is considered to beacid-resistant according to present day standards when it falls into thecategory of class A or better when tested "by the Test For AcidResistance of Porcelain Enamels, part 1Flatware, Bulletin T-7, PorcelainEnamel Institute, 1946. The enamel compositions of the presentinventionin general possess at least class A acidresistance,-aud the preferredenamels possess even better acid-resistance, including classes A+, AA.and AA.

'lrrthe case of alkali-resistance, the presently favored test fordetermining alkali-resistance consists of subjecting a known area ofsurface of enamel to a boiling 5 sodium hydroxide solution under refluxconditions for sixhours. The resulting loss in weight is reported asmilligrams per square inch. We have adopted a figure of 20 milligramsper squareinch as a suitable dividing point under these severeconditionsof test. That is to say, those enamels which result in a loss of weightof no more than 20 milligrams per square inch under the stated testconditions are considered to possess alkalire'sistance. All of theenamels of the present invention possessan'alkali-resistance of lessthan 20 milligrams pervsquare inch, and the preferred compositionspossess muchhigher resistance to alkali, on the order of Ste 10 Theenamel compositions of the present invention are the first enamels, sofar as is known, which possess these combined properties ofsuper-opacity, acid-resistance and alkali-resistance. In additiontothis, however, the present:

enamel compositions can be fired at moderately lowteme peratures, in theneighborhood of l450-l-500 F. so that they can readily be applied tolighter gauge metals and broader aspects of the present invention, willcomprise, in

terms of oxide analysis, oxides of atleast two of the.

alkali metals sodium, potassium and lithium; boron oxide. (B zirconiumoxide (ZrOg); silica (SiO fluorine, (F) andv titanium dioxide (TiOwithin certainproportional ranges. Referring tothe alkali metal content,the total amount of alkali metal oxide present is established by theworkability, expansion characteristics and appear-- ance ofthecompositions and the balance ofthe variousother oxide constituentsrequired for the above-discussed:

advantageous properties. Accordingly, the total alkali.

is lowered and the color-stability and firing stability of the enamelare poor.

The enamels of the present invention are titania-opacified enamels, thatis to say the high opacity provided in the final enamels is due toprecipitation of titania during the enamelling operation. Thestitania isan integral part of the frit, being dissolved therein, and is notincorporated with the frit as-an additive: duringmilling; To: providethe high opacity mentioned. above for the present enamel. compositions,,the1TiO content must be at least about 16% and may go as. high. as about23 In the. preferred compositions, the TiO content is between about 1'8and about 21% metal oxide contentof the present enamels should not. fallbelow about 11.2 not exceed about 18.3%, and in. the.

preferred compositions the total alkali metal oxide content Will bebetween about 13' and about 16%. As stated,,.

in the compositions of the present. invention oxides of. at least two ofthe alkali metals will be present, that is. to say the compositions willcomprise at least oxides of. so-. dium and potassium, oxides of sodiumand lithium, or

oxides of. lithium and potassium. In accordance. with pre-:

ferred practice, at least one of the alkali metal oxides. presentwill belithium oxide, since the presence of lithium. oxidel has been found. toinfluence substantiallythe: acidresistance of the enamel and themoderately low firing. temperature thereof. The presence. of the oxidesof the three stated alkali metals has been found to be particularlyadvantageous. In order to obtain the most. advantagieous results from acombination of two or more of the: alkali metal oxides, it has. beenfound that each of them should be present, in an amount of at leastabout 1% of the enamel composition. The boron oxide (B 0 con tent of thepresent enamels should remain within the range stated above in order toprovide proper workability combined with high opacity, alkaliandacid-resistance. Thus, with amounts of B 0 less than about 3.9% theworkability of the enamels falls off sharply whereasv With- As statedabove, the" enamel compositions may and preferably do contain smallamounts of at least one of the divalent oxides selected from the groupconsisting of calcium oxide (CaO), barium oxide (BaO), magnesium oxide(MgO),.str.ontium,oxide- (SrO)., lead oxide (PbO) and zinc oxide (Z110),phosphorus. pentoxide (P 0 and alumina (A1 0 Thepresence. of the.divalent ox? ides. in. the present compositions, although notrequired,,. is highly beneficial, and thus amounts. thereof. up toabout.

718% are generally included. Above about 7.8% of di valent oxide. the.acidrresistanee and workability ofthe enamels, fall ofif sharply. Of thedivalent oxides, ZnO and.

combinationsof ZnO and MgO, are preferred. In the,

preferred. compositions ZnO or a-combination of. ZnO" abeur;4.9'% 01310,. In the preferred compositions P 0 inc'lud'ed inan amount betweenabout 0.8 and about: 2.9%. The pres nce. of a small amount of A50 docs.net deleteriously, affect. the valuable properties discussed herein.sineeithe present enamels possess .ahi'gh. toler amounts above about17%.the alkali resistance. is reduced substantially In the preferredenamel compositions, for optimum workability and alkali-resistance, theB 0 is-in an amount between about 7.9 and about 11%. The pres.- ence ofzirconium oxide (ZrO inv proper amounts cone trols the acid-resistanceand alkali-resistance. of the enamels, and it has been found thatamounts below about 0.5% result'in sharply reduced alkali-resistancewhereas amounts above about 10% result insharply reduced workability.Hence, these figures establish the range ofj proportionsfor Z rO in thepresent compositions generally. In the preferred enamel composit-ions,the ZrO is in an amount between about 2 and about 6%. Since the:compositions of the. invention are essentially silicates, the silica(SiO is the chief acid constituent, and the amountthereof in thecompositions is dictated by this. fact as well as by the proportions of?the other necessary constituents required to provide the advantageousproperties, discussed herein. The. SiO will generally range between;about 37.5% and about 56.6%., and, in the preferred, compositions,between about Hand about 52%. It has been found that the. fluorine (P)content of theprescnt compositions has amarked bearing on'theworkability-,; acid-resistance, color-stability and firing temperature.At

fluorine contents. below about 1.8%, the workability! and;

mice for A150 less expensive raw materials supplying one. of more of theother constituents and containing thisv material may be employed for thesake of convenience and economy. Above about 4'% Al O however, acid-.resistance falls off sharply. Hence, the compositions of thepresent'inventi'on will generally include no more than about 4% of A1 0and, in the preferred. compositions, A150 will be present in an amountbetween about, 0.5, and about. 2%

. The enamel compositionof the present invention. willconsist'essentially of the constituents discussed above in. thestatedamounts. This does not mean, however, that small amounts of othermaterials not deleteriously altering the. advantageous characteristicsof the enamel are excl'uded'. For example, small amounts of SnO ,v Sb OMoog, 05 0 and the like, such as in amounts up. to. about 1%,,do notsignificantly deleteriously alter thede-f sirable characteristics of theenamel, and may in some. cases, be desirable for color control.

The.v compositional ranges. and amounts referred to.

above are in. terms: ofv percent by weight, and refer to the.

proportions of the constituents in the frit and not to the; compositionof the final enamel inasmuchas these may be altered somewhat due. to theaddition of certain compounds to the frit in making up the enamelcoating: composition.v

Thercomposi-tion, in the form of a. frit,. having; the above rdescribed.chemical composition, may be prepared; in accordance with conventionalpractice. Asis well. known,; in preparing the frit, compounds ultimatelyproviding the; desired oxide analysis+in the present case: providing.thea-ahove-described oxide analysis-'-are. mixed togetherin; fineparticle. size and melted, and the molten massquickly cooled, as bywater quenching, and dried:

. Compounds which may be employed'to provide-the above chemical-oxideanalysis are well known in thefart, and

the provisionof the-above-deserib'ed oxideanalysiswill present noprohlem to those skilledin the vitreous enamel art. Thus, the alkalimetal oxides may be provided, for example, by carbonates, phosphates,nitrates,

erals containing silica as well as one or more of the desiredconstituents. The fluorine may be provided by any of the alkali metalfluorides, such as sodium fluoride,

lithium fluoride, and the like, and more complex fluorides, for instancesodium silicofluoride, and the like. It will be understood, of course,that during melting in the preparation of the frit, a certain amount offluorine is lost due to volatilization, the exact amount depending up onthe fritting procedure followed. The amount of the :source of fluorine,therefore, will be selected to provide :a theoretical fluorine contentin the frit as specified above. valent oxides may be provided bysuitable salts, such as carbonates, or the oxides themselves may beusedjj The P may be provided by suitable phosphates, such as.

one of thosementioned above. The A1 0 will normally be provided by amineral supplying one or more of the constituents mentioned above. Onecompound may provide some or all of a plurality of the above-mentionedoxides, for example, sodium phosphate may be relied upon to provide Na Oand P 0 In any event, the compounds will be selected in accordance withwell known practice to provide the proper oxide analysis, and in theevent such compounds contain a constituent not desired in theultimatefr'it, such constituent will be of a volatile nature so that itmay be removed during heating and melting of the mixture. For example,where carbonates are employed, carbon dioxide is liberated, and whereammonium compounds, such as ammonium phosphate are employed, the ammoniais liberated.

The materials, in fine particle size, providing the desired chemicalanalysis upon heating and melting thereof, are mixed together inaccordance with common practice, and heated to an elevated temperatureto provide a molten, pourable mass. With the compositions of the presentinvention, temperatures in the neighborhood of from about 2200 to about2350 F. may be employed to provide the molten mass. The molten mass isthen quickly chilled, such as by pouring it into a water bath, and suchquick chilling causes the glass-like mass to fracture into small pieces.These small pieces are recovered and dried. i The resulting frit is toocoarse for direct use, as an enamelling composition and may, therefore,be ground to the desired size in accordance with well known practice,Additives of thetype normally incorporated in vitreous enamellingcompositions may be added to the frit. For example, it is normally thepractice to incorporate a suspending agent in the composition, usuallyduring milling of the frit. These suspending agents, such as enamellingclays, bentonite, sodium silicate, and the like, may be added to thefrit either as a solid or as a dispersion in water. Similarly, anelectrolyte salt is also normally added to the composition to flocculatethe composition when Water is added thereto controlling the viscosity ofthe resulting slip. Examples of such salts are thewatersoluble--*particularly the alkali metal--chl0rides, carbonates,nitrates, phosphates, and the like. A preliminary binder impartingstrength to the coating until it is fired, such as gum tralgacanth,mayalso be included in the composition. As stated, in the composition ofthe present invention, the titania in the frit serves as the ultimate.opacifying agent. Hence, .it is not necessary to The TiO is provided bytitania itself. The di-.

6 add an opacifying agentto the flit, such as during milling.

The milled composition comprising the frit and the desired additives andsufiicient water to provide the slip is :then applied to the desiredmetal surface, as by spraying, brushing, or the like. The coating isthendried and: fired to cause the coating to fuse and to vitrify into acontinuous white, opaque, glassy coating. The firing of the coatingfollows well known procedures; however, as stated, an important featureof flte compositions of the present invention is their ability to befired at moderately.

low temperatures, in the neighborhood of 1450-1500 F. Many of theenamels may even be fired at temperatures as low as about 1420 F. l

The compositions of the present .inventionand their preparation as wellas their advantageous properties will be more readily understood fromconsideration of the following specific examples, which are given forthe purpose of illustration only and are not intended to limit the scopeof the invention in any way.

EXAMPLES r-xxxrx Thirty-nine frits, having different theoreticalcompositions as set forth in the tables, are prepared. below followingconventional practice but employing compounds and materials ultimatelyproviding, after melting and fritting, the theoretical oxide analysisset forth in the tables. The

clear melt formed in each case is poured into water to quench andshatter the glass into easily friable particles. The water is drainedfrom the solids, and the frit is then dried. Each frit is prepared intoa, standard mill consisting of:

The mill mixes are ball milled for about four hours until 2 grams (:1gram) of residue remainon a 200 mesh screen when 50 cc. of slip areWashed through the screen. Each slip is then removed from the ball mill,and sprayed onto one side of a ground-coated sheet steel panel at therate of 25 grams of dry enamel per square foot of surface. Each panel isthen fired at l450-1500 F. Opacity is measured on a Hunter Reflectometerusing three color filters. Acidand alkali-resistance tests are made onthe normal fired specimens in accordance with the tests describedpreviously herein. Acidand alkali-' resistance and opacity are set forthin the tables for each of the compositions.

Tables I II III IV V VI 5.06 5.06 5.06 5:06 1 5. 06 i 5. 06. 4.69 a 4.694.69 4.69 4:69 4.69 3.58 3.58 3.58 3. 58 3.58 3.58 2. 91 2, 91 2.91 2:912: 91 2. 91 0. 88 0.88 0. 88 0.88 0. 88 0.88 1. 81 1. 81 1.81 1. 81 '1.81 0.00 2.01 4.02 6.03 8.04 10.05 6.03 1 0 0 0 o 0 v0 20. 25 20. 25 20.25 20. 25 20:25 20. 25

AA AA AA- A AA Alkali Resistance, r t

mg.lsq. in 18.6 13.1 8.86 7. 41 6.04 8.99 Green Reflectance, 25 gmsJsq.it. Applicationnnn. 76.8 75.8 77.8 78.3 77.4 77.6

Tahles Continued VII VIII IX X XI S101; 46. 80 4 44.79 42; 78 40. 77 42.77 320:. 7. 99 7. 99 7. 99 14. 02 12. 02' K;O I 5.06 5. 06 5.06 5.065.06 N950; 4. 69 4. 69 4.69 4. 69 4. 69 L110. 3. 58 3. 58 3. 58 3. 58 3.58 F 2. 91 2.91 2. 91 2. 91 2. 91 P 0. 88 0. 88 0.88 0.88 0. 88 ZnO.. 1.81 3.82 5.83 1.81 1.81 2101.. 6. 03. 6.03 6.03 6.03 6.03 A1205. 0 0 0 00 T101... 20.25 20. 20.25 20. 25 20.25 Acid Resistance AA 'AA- A A+ AAlkali Resistance, mgJsq.

in.- 7. 22 5. 28 8'. 91 I6. 40 10. 0 Green Reflectance, 25 gms./

sq. ft. Application 78. 5 82.4 82.1 80.1 79.4

XII XIII XIV XV XVI S101 48. 81 45. 79 45. 79 47. 81 44. 81 B201- 5. 988.16 8.16 7. 99 7. 99 10-. 5.06 5.49 5.00 4.68 5. 86 Nero. 4. 69 5. 114. 60 4. 33 5. 35 Li;0-- 3. 58 3. 01 4.01 3. 31 4.11 F 2; 91 2. 97 2. 972. 91 2. 91 R05. 0. 88 0. 89 0. 89 0. 88 0. 88 ZnO.. 1. 81. 3. 91 3.91 1. 81 1. 81 Z101. 6.03 6.16 6. 16 6.03 6.03 A1105. 0 0 0 0 0 TiO20.25 18. 51 18. 51 20. 25 20. 25 Acid Resistance..- AA AA AA AA AAAlkali Resistance,

9. 90 10. 3 8. 9 7. 53 8. 10 Green Reflectance,

sq. ft. Application 82.8 76. 7 82. 7 71. 0 84.12

XVII XVIII XIX XX XXI XXII 43. 79 45.80 41. 78 44. 79 41. 78 41.67 7. 997.99 7. 99 7. 99 10.01 10.01 6. 21 4. 68 6. 21 5. 06 6. 20 8. 5. 74 4.83 5. 74 4. 69 5. 74 8. 04 4. 39 8. 31 4.39 3. 58 4. 39 0 2. 91 2. 91 2;91 2:91 2. 91 2. 91 0. 88 2. 89 2. 89 2. 89 0. 88 0.88 1. 81 1. 81 1. 811.81 1. 81 1. 81 6. 03 6.03 6. 03 6.03 6.03 6.03 0 0 0. 0 0 0 20. 25 20.25 20. 25 20. 25 20. 25 20. 25. AA AA A+ AA A-l- A+ AikelLResistence.

mgJsq. in. 7. 64 10.2 10. 8 11.1 12.7 13.0 Green Reflectance,

25 gins/sq. it. Application 85. 2 70. 6 83. 0 77. 8 85. 6 72. 2

XXIII XXIV XXV XXVI XXVII XXVIII AA A+ A+ A A AA Alkali Resistance,mg./. sq. in 8. 63 8.02 6. 75 10. 05 8.19- 1731 Green Refiec V tance, 25gms./sq. it. 0 Application 76:4 79. 1 7879 78. 3 77. 5 81. 5

XX X XXX XXXI XXX XX XX 3.50 3.58 2.00 3.19 3.50 3.50" 2.89 2.91 2.89 2.89 2.89 2. .9" 0.87 0. 88. 0.87 0.87 0. 87 0.87 1.80 1.81 1.80 1.80 1.801.80 5.00 4. 02. 6.00 6.00 3. 00 10.00" 2.00 0.67 0 0 3.00 0 20.06-20.25 20.06 20.06 20.06 20.06 Acid'Resistance. A AA A A A A AlkaliResistance,mg./ sq. in 9. 75 9.85 13. 77 17.65 16.15 7.11 GreenReflectance, 25 gnis/sq. ft. Application". 80.03 77.9 71.0 76.0 81.477.2

XXXV XXXVI XXXVII XXXVIII XXXIX 7. 87 8. 88 12.79 12. 76 A AA AAReflectance 82. 4 88. 6 74. 4 S0. 0 79. 8

Considerable modification is possible in the selection of the particularcombination of constituents and amounts thereof without departing fromthe scope of the present:

invention.

We claim:

1. A vitreous enamel composition in the form of a {tit consistingessentially of between about 13 and about 16% of the oxides of at leasttwo of the alkalimetalsselected from the group consisting of sodium,-lithium and v potassium, between about 1.8 and about 5.8% of at. leastone of the divalent oxides selected from the group consisting of CaO,BaO, MgO, SrO, PhD and ZnO, between about.

7.9 and about 11% of B 0 between about 0.8 and about" 2.9% ofP O betweenabout 2 and about 6% of Z10 between about 44 and about 52% of SiObetween about 2.5 and about 4.9% of fluorine and between about 18 andabout 21% of TiO said percentages being on the weight basis.

2. The frit of claim 1 wherein one of the alkali metal prises ZnO.

8. The frit of claim 3 wherein the divalent oxide cornprises ZnO.

References .Cited in the file of thispatent.

UNITED STATES PATENTS 1,949,479 Kinzie et a1. Mar. 6, 1 934 1,988,800Kinzie et a1. Jan. 22', 1935' V FOREIGN PATENTS

1. A VITREOUS ENAMEL COMPOSITION IN THE FORM OF A FRIT CONSISTINGESSENTIALLY OF BETWEEN ABOUT 13 AND ABOUT 16% OF THE OXIDES OF AT LEASTTWO OF THE ALKALI MEATALS SELECTED FROM THE GROUP CONSISTING OF SODIUM,LITHIUM AND POTASSIUM, BETWEEN ABOUT 1.8 AND ABOUT 5.8% OF AT LEAST ONEOF THE DIVALENT OXIDES SELECTED FROM THE GROUP CONSISTING OF CAO, BAO,MGO, SRO, PBO AND ZNO, BETWEEN ABOUT 7.9 AND ABOUT 11% OF B2O3, BETWEENABOUT 0.8 AND ABOUT 2.9% OF P2O5, BETWEEN ABOUT 2 AND ABOUT 6% OF ZRO2,BETWEEN ABOUT 44% OF FLUORINE AND BETWEEN ABOUT 2.5 AND ABOUT 4.9% OFFLUORINE AND BETWEEN ABOUT 18 AND ABOUT 21% OF TIO2, SAID PERCENTAGESBEING ON THE WEIGHT BASIS.